Now, a near perfect GPA at a state school is a good thing. Is this a state school that has an accounting PhD program? Do you know who the top researchers are there? Is it a teaching or research school? These differences matter.
It sounds like you want to put a couple years of prep in in order to get into a top school. If your alma mater is a research university and you have good connections there, you may be able to get some sort of gig as a research assistant while taking extra classes. This would be a great way to spend your time. The problem is that with just an accounting undergrad and a MACC, you aren't much good as an assistant because you likely can't program anything and you don't understand the economics stuff that well. So you can do data collection from company filings or things like that. You may also make a good TA. This would keep you in the University environment, but not do too much good for you as far as admissions go.
I think it is unlikely that you will be able to fill a schedule with useful classes and you may be overestimating the amount of math that you should really do. Unless you want to do theory work, calc 1-3 and linear algebra is enough for most schools. i.e. I know someone that got into Wharton with that math, a finance undergrad, and an MBA. Calc 1-3 need to be taken sequentially and linear algebra can be thrown in there at the same time as 2 or 3 depending on the school. Math stats and probability require multivariate integration, which is a calc 3 topic, so they usually have to come after calc 3. You should be able to take some sort of intro econometrics without calculus, but the calc based stuff is good and at the PhD level you do measure theory things which still hurt my brain. So, as far as math goes, you are probably better off trying to get calc and linear algebra done part time in the next year. Switching to corporate would help with this. You probably also want at least intermediate microeconomics. Business calc should have you prepared for this at most colleges. You will likely need to do partial differentiation, but that is easy to learn if you forgot it.
Volunteering as an RA is a good idea, but it will likely be data collection and clean up, which isn't the fun part of the job. If you are still near a research university you should seriously consider sitting in on a seminar and attending the workshops. This will give you a better idea of what you are getting into than anything else and provide great background. It will help you select schools you want to go to, define research interests, earn good letters, etc.
As for targets. R1 universities don't vary too much in pay, and a lot of state schools are in much cheaper areas than the top private schools. I am at a big state school and we have placed students making over 200 base since I have been here. The low end of what you can really expect is like 150 if you get a placement at a more teaching heavy school. I don't think there is as big of a separation as you are envisioning. I also had an offer from a top private B-School and the research environment is just as good here, if not better. The real line you should look at is teaching vs. research schools. So basically, you should apply to places where people are doing research that you are interested in. I can answer more questions on this if you give a little more detail, I just don't want to write an essay write now.